Medial compartment arthritis of the knee is caused by uncompensated horizontal bending stresses coming from the direction of the medial side of the knee. These stresses produce an overload in the medial compartment with subsequent erosion of the joint surface. Eventually, the bone beneath the joint collapses so that the femur and tibia loose their normal angulation producing a condition commonly known as bow legs. Each person has their own individual set of factors that are responsible for the uncompensated horizontal bending stresses.
Surgical treatment of this arthritis requires that the push-pull forces on the knee first be measured since too little correction will leave the patient bow legged, while too much correction will result in knock knees, the opposite condition. The ideal position is that where the forces through the knee are balanced during weight bearing.
The treatment has two essential goals. The first is to move the angles of the femur and the tibia to a predetermined correct position where the desired distribution of load between the medial and lateral compartments of the knee is obtained. The second is to restore the joint surfaces to proper contact.
Previous attempts to correct the condition involved performing a simple lateral osteotomy in which a wedge of bone is removed from the lateral or outer side of the tibia and the resulting two surfaces are brought together to straighten the leg. However, simply correcting the angle of the tibia relative to the knee and the femur is not sufficient since the angulation of static deformity in a malunited long bone is not equivalent to the angulation of a dynamic deformity at the level of the knee. The knee is a weak cleavage plane in an otherwise rigid column running from the hip to the ankle. An imbalance of the push and pull forces exerted on this plane will produce variable combinations of angulation of the femur as compared to the tibia and each such combination will have a different effect on the surface of the knee joint. The lateral osteotomy procedure ignored the forces affecting the knee as well as the congruency of the convex/concave curvatures of the joint surfaces. Thus, the lateral osteotomy fell into disrepute due to the prevalence of post operative complications such as foot drop due to paralysis of the peroneal nerve and vascular problems in the leg. In addition, the eventual angle in the leg was impossible to predict.
The lateral osteotomy procedure is represented by U.S. Pat. No. 4,936,844, Chandler, et al., which is related to a bone fixation system for securing the osteotomy surfaces together.
As the lateral osteotomy procedure began to fall from favor, total knee replacement became popular as a way to correct arthritic conditions in the knee. Although initially used for cases where arthritis was so extensive that it involved the entire knee, subsequently, the indications for total knee replacement were extended to include patients with bow legs and knock knees in which arthritis was limited to only the medial or lateral compartment.
Although successful, total knee replacement involves significant surgical time as well as post operative recuperation such that a simpler method of dealing with the limited conditions of bowleggedness and knock knees was desired. In the case of bowleggedness, another look at tibial osteotomies was taken with the result being the development of the medial osteotomy in which an osteotomy is made on the medial, or inner, side of the tibia and is opened up and secured, thereby straightening the leg. In early procedures, a graft of bone taken from the patient's iliac crest was inserted into the osteotomy.
Later procedures involve the use of blocks and wedges to hold the osteotomy open. However, these are not without their own deficiencies. For example, U.S. Pat. Nos. 5,620,448 and 5,749,875, Puddu, present a bone plate system for proximal tibial osteotomy in which a tibial wedge is resected and a calibrated wedge tool is employed to open the osteotomy to the desired degree. The bone plate having a small protruding block is secured in place, the block serving as a prop to hold the osteotomy open. The rest of the osteotomy is packed with a bone graft. A significant problem with this system is that the osteotomy is subject to settling or compression within the open wedge since the support obtained from the plate/block combination is limited to a small area of the outer edge of the osteotomy. Settling can result in crumbling of the graft and anterior/posterior and/or lateral rocking through the osteotomy which interferes with the establishment of a secure graft and which places stress on the plate and screws further weakening the area.
In an alternative, U.S. Pat. No. 5,766,251, Koshino, presents a wedge-shaped spacer for use in corrective osteotomies. Although longer than the depth of Puddu's block, the wedge of Koshino is of a relatively narrow width. The result of this is that, like Puddu, the osteotomy is subject to settling on either side of the wedge which can lead to rocking and weakening of the graft through the osteotomy. The wedge has the added disadvantage of being of a length such that it would interfere with a subsequently performed total knee replacement, necessitating an additional procedure to remove the wedge. Furthermore, it has been found that the compression stresses through the osteotomy are such that use of a wedge is substantially limited to correction angles of 10.degree. or less to avoid crumbling of the graft and expression, or squeezing out, of the implant.
In addition to their mechanical disadvantages, these medial procedures, like the lateral osteotomy, fail to take into account the push-pull forces through the knee when calculating the correct placement and corrective angle for the osteotomy. The result of this is that the condition frequently reasserts itself after a relatively short period of time. The longevity of the medial osteotomy procedure and the knee joint is dependent upon achieving congruence of the joint surface. This includes establishing an even distribution of load between the medial and lateral compartments, establishing the direction of stresses on the joint in as vertical an orientation as possible, utilizing all available contact area in the joint to spread the load, and maintaining ligament tension to prevent rattling of the joint during motion. Only by establishing and balancing the push-pull forces will the osteotomy be successful over the long term to improve unicompartmental arthritis of the knee and to correct tibial displacement.